A linear antenna, a printed antenna on a circuit board, a patch antenna, electromagnetic antenna comprising a magnetic substance and a coil wound around the magnetic substance are usually employed, these antenna are categorized as an electric filed type or a magnetic field type.
Usually a bar (linear) antenna, categorized as the electric field type, radiates from the electric field. However, in a high frequency band the radiation is deteriorated as an impedance of the antenna becomes zero by weakened electric field by metal surfaces, water, humans and animals. The electromagnetic antenna categorized as the magnetic field type, may enhance its efficiency by excitation of magnetic fluxes on the metal surface induced by an image effect. However, in a high frequency band such as VHF, the antenna cannot enhance its efficiency due to a deteriorated specific permeability, which means the magnetic substance core effects are not attained, as a result, an antenna efficiency cannot raised.
A slot antenna, categorized as the electric field type, having a slit or a slot formed in a metal surface, also utilized surface current and electric field. However, when there is a metal surface at rear side of the antenna, electric field generated in the slot area is compensated by the metal surface at the rear side, so that the slot area does not radiate.
A dipole antenna also has the same problem as the slot antenna. However, the dipole antenna can radiate symmetrically radiate in 360° around poles of the dipole antenna, while the slot antenna radiates only in upper direction and lower direction when the metal plate is placed horizontally. Utilizing these features as well as electric current and electric field, the metal surface itself can utilize as a part of the antenna by flowing metal surface electric current by induced current generated strong electric filed around a hollow slot formed in the metal surface, of which length is ca. half-wave.
The following technologies and patents are disclosed as prior arts.
An antenna comprising an IC attached to a slot, a metal plate to which a dielectric is sandwiched. In some cases, a thick dielectric is used in order to alleviate effects of the metal surface. A short circuit is formed at one side of such antennas, in order to alleviate effects of the lower surface of the metal surface, but since an area around the slot is open, effects of the lower metal surface cannot eliminate completely. When there are no metal surfaces, electric current path becomes short due to radiation from the slot area in one direction, so that excitation of the electric field is unfavorable. When the antenna receives radio waves, a voltage induced by received the radio waves is low, so that an excited voltage at the slot is low. A slot antenna for metal having a length of ¼ wave with a high impedance is available, but there are problems of impedance matching and radiation efficiency.
A composite antenna comprising a conductive antenna and a magnetic antenna is available, but such composite antenna has a complicated structure.
Patent applications relating to various slot antennas were filed, which are as follows: a composite antenna comprising a slot and a dipole, a waveguide type antenna having a slot, a slot antenna having a slot with ½ or ¼ wave.